2Vapour-Liquid Equilibrium for Pure Substancerefers to the state of coexistence between liquid and vapour phases concerned with the conditions of Tand Pat which the phases coexist and the distribution of components between the phases at equilibrium phase transitions are represented by phase diagrams consider the phase diagram for a pure substance (see figure 1) vapour pressure: the pressure exerted by a vapour at equilibrium with a solid or liquid phase the vapour pressure of a pure substance is a function of Tonly Fig 1: Phase diagram for a pure substance

3The Phase Rulethe phase rule for non-reacting systems is expressed as NF2(1) where F= number of independent intensivevariables (or degrees of freedom) = number of phases in equilibrium N= number of components the total set of intensive variables for a system at equilibrium includes T, Pand N-1 mole fractions for eachphase the phase rule defines the number of variables which must be specified from this set in order to fix the values of the remaining intensive variables Example 1For a pure component with L-V coexisting phases, F= 2 - 2 + 1 = 1. Therefore Pis fixed once Tis specified. This is in agreement with the fact that the vapour pressure of a pure liquid only depends on T. Since each value of Tyields a single value of P, the L-V coexisting phase project as a line in the P-Tplane. The triple point (S-L-V coexisting phases) projects as a point since F= 0 and neither Tor Pcan be arbitrarily specified. Example 2For a binary system with L-V coexisting phases, F= 2 - 2 + 2 = 2. If we use xto represent liquid composition and yfor vapour composition, the total set of intensive variables is T, P, x1and y1. Thus for given values of Tand Pthere is only one set of values for the vapour and liquid compositions. All equilibrium states of the system can therefore be represented in a three-dimensional P-T-xyphase diagram.

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